1. Technical Field
The present invention relates to seals for gas sealing in solid polymer electrolyte fuel cells, and relates to a forming method therefor.
2. Related Art
In solid polymer electrolyte fuel cells, a separator is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The membrane electrode assembly is a layered structure, in which a polymerized electrolytic membrane is held by a positive catalytic electrode (cathode electrode plate) and a negative catalytic electrode (anode electrode plate), and a gas diffusion layer is layered on the outer surface of each catalytic electrode. The separator is made from a material having electron transmitting characteristics, and has plural grooved gas passages in which a fuel gas such as hydrogen gas, an oxidizing gas such as oxygen or air, and a coolant flow individually. The separator is layered on the membrane electrode assembly such that linear protrusions between the gas passages are contacted with the gas diffusion layer.
According to the fuel cell, a fuel gas is provided to the gas passage of the separator at the negative electrode side, and an oxidizing gas is provided to the gas passage of the separator at the positive electrode side, whereby electricity is generated by electrochemical reaction. During the operation of the fuel cell, the gas diffusion layers transmit the electrons generated by the electrochemical reaction between the catalytic electrode layers and the separators, and diffuse the fuel gas and the oxidizing gas. The catalytic electrode layer in the negative electrode side results in a chemical reaction for the fuel gas so as to generate protons and electrons. The catalytic electrode layer in the positive electrode side generates water from oxygen, the proton, and the electron, and the polymerized electrolytic membrane facilitates ionic migration for the proton, whereby the electric power is provided via the positive and negative catalytic electrode layer.
In the above-described fuel cell, the fuel gas, the oxidizing gas, and the coolant must be flowed in the individual gas passages, so that the gas passages are separated from each other by a seal. The sealing portion varies according to the structure of the fuel cell stack. For example, a seal is provided around a communicating opening of the gas passages penetrating the fuel cell stack, around the membrane electrode assembly, around a coolant passage provided on the outer surface of the separator, and around the circumference of the outer surface of the separator.
According to conventional sealing technology, in general, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, is formed into a shape of a sheet or an O-ring, and is mounted to a sealing portion. The sealing member seals the sealing portion by a reaction force generated by being compressed in a stacked condition. As other sealing structures, a seal in which an inorganic material formed by carbon or ceramics is compressed, a mechanical seal using caulking, adhering, and the like have been provided.
Fuel cells are often carried or installed in automobiles for use. In these cases, the cells are stringently required to be small and thin. Since separators are usually made from brittle carbon, they are readily broken during assembling of a fuel cell stack. Therefore, seals made from organic rubbers are widely used, since they are flexible and have suitable reaction force, thereby preventing breakage of the separator in the assembly a fuel cell stack.
FIG. 6A through FIG. 6E show a related method for providing a seal, which is made from an organic rubber and tightly contacts with a separator (not shown), over the circumference of the membrane electrode assembly 1. The membrane electrode assembly 1 is formed such that a polymerized electrolytic membrane 4 is held by a cathode electrode plate 2 and an anode electrode plate 3, and a gas diffusion layer 5 is layered on the outer surface of each electrode plate 2 or 3. The center electrolytic membrane 4 has a larger area than that of each electrode plate 2 or 3 and each gas diffusion layer 5, and the circumference 4a thereof projects from them. As shown in FIG. 6E, a seal 60 is integrally formed with the circumference 4a. Reference numerals 70 and 80 in FIGS. 6A through 6E are an upper die and a lower die of a forming die. In the forming die, recesses 71 and 81 into which the membrane electrode assembly 1 is fitted and grooves 72 and 82 forming a cavity 90 are formed in vertical symmetry. A gate 73 communicated to the cavity 90 from outside is formed in the upper die.
In order to provide the seal 60 to the membrane electrode assembly 1, first, as shown in FIG. 6A, the membrane electrode assembly 1 is fitted into the recess 81 of the lower die 80, and next, as shown in FIG. 6B, the upper die 70 is lowered and the membrane electrode assembly 1 is fitted into the recess 71 of the upper die 70. Then, as shown in FIG. 6C, the upper and lower dies 70 and 80 are clamped to each other, and next, as shown in FIG. 6D, a sealing material 60A is charged into the cavity 90 from the gate 73. The sealing material 60A is vulcanized when the material is a vulcanizing rubber, is heated when the material is of the heat-cure type, or is then removed from the opened upper and lower dies 70 and 80 without heating when the material is of the thermoplastic type, a membrane electrode assembly 1 as shown in FIG. 6E is then obtained. The electrolytic membrane 4 of the membrane electrode assembly 1 is integrally formed with the seal 60 which surrounds the electrode plates 2 and 3, and the gas diffusion layer 5.
The thickness of the membrane electrode assembly 1 is not strictly constant, and is sometimes thicker or thinner than the regular thickness. According to the forming method shown in FIGS. 6A through 6E, when the thickness of the membrane electrode assembly 1 is in the regular range, suitable tightening thickness of the seal, namely, a sealing pressure and a sealing height can be obtained. However, when the thickness of the membrane electrode assembly 1 exceeds the regular range, as shown in FIG. 7A, the circumferences of the upper and lower dies 70 and 80 do not contact sufficiently with each other and form a clearance S1, into which the material for sealing inserts. As a result, as shown in FIG. 7B, a burr 61 is formed at the outer circumference of the seal 60, and the thickness of the seal 60 is greater than the regular range thereof, and the burr must be removed.
In contrast, when the thickness of the seal 60 is thinner than the regular range, as shown in FIG. 8A, a clearance S2 is formed between the upper die 70 and the membrane electrode assembly 1, the seal 60 is formed without compression of the membrane electrode assembly 1. As a result, as shown in FIG. 8B, the height H2 of the seal 60 from the membrane electrode assembly 1 is greater than the regular range. Therefore, the seal 60 is excessively compressed when a fuel cell stack is assembled, so that the separator is damaged or deformed. Furthermore, the material for sealing may flow into the clearance S2 between the upper die 70 and the membrane electrode assembly 1 so as to damage the membrane electrode assembly, and the sealing material may adhere to the membrane electrode assembly 1, so that the power generation performance may be deteriorated or energizing may be impossible in some cases.
Therefore, an object of the present invention is to provide a seal for fuel cell and a forming method therefor, in which constant sealing pressure and sealing height can be obtained even if the thickness of the membrane electrode assembly varies, whereby problems such as damages to members such as separators closely contacting with the seal or leakage of the seal can be inhibited in the condition in which membrane electrode assemblies are assembled to a fuel cell stack, so that excellent sealing performance can be obtained.
The invention provides a seal in a fuel cell for sealing a membrane electrode assembly comprising: a pair of catalytic electrode layers, a polymerized electrolytic membrane held between the catalytic layers, and a gas diffusion layer layered on the outer surface of the catalytic electrode layer; wherein the seal is integrally formed with the polymerized electrolytic membrane, the height thereof is greater than that of the gas diffusion layer, and the difference between the heights of the polymerized electrolytic membrane and the gas diffusion layer is constant.
According to the seal in the invention, the height of the seal with respect to the membrane electrode assembly is constant even if the thickness of the membrane electrode assembly varies. Therefore, in assembling a fuel cell stack by stacking the membrane electrode assemblies with a predetermined assembling pressure, the sealing pressure and the sealing height is constant in accordance with the assembling pressure. As a result, problems such as damage of the separator closely contacting with the seal and gas leakage due to insufficient sealing pressure can be inhibited, whereby excellent sealing properties can be obtained. Since the seal is integrally formed with the electrolytic membrane, the number of parts can be reduced, and handling for assembling is facilitated and assembling accuracy can be improved. The seal in the invention is integrally formed with the electrolytic membrane as mentioned above. Specifically, the seal is preferably adhered to the electrolytic membrane by vulcanizing adhering or melting adhering. That is, the portion adhered by vulcanizing or melting is superior in adhesion properties, so that permeation of gas, and the like, at adhered portions can be completely prevented.
The invention provides a method for forming a seal in fuel cell for sealing a membrane electrode assembly comprising: a pair of catalytic electrode layers, a polymerized electrolytic membrane held between the catalytic layers, and a gas diffusion layer layered on the outer surface of the catalytic electrode layer; wherein the membrane electrode assembly is set in a forming die having a cavity in which the seal is integrally formed with the polymerized electrolytic membrane; the die is clamped so as to load predetermined pressure on the membrane electrode assembly; and a material for the seal is charged into the cavity.
According to the forming method of the invention, since predetermined pressure is loaded on the membrane electrode assembly in clamping the dies, the membrane electrode assembly is usually compressed by the dies, so that the height of the cavity of the seal, namely, the difference between the height of the seal after forming and the gas diffusion layer of the membrane electrode assembly is usually constant independently of the thickness of the membrane electrode assembly. Therefore, the sealing pressure and the sealing height are constant in assembling the fuel cell stack even if the thickness of the membrane electrode assembly varies. As a result, problems such as damage of members closely contacting with the seal and leakage due to insufficient sealing pressure can be inhibited, whereby excellent sealing properties can be obtained.
It should be noted that the predetermined pressure is preferably approximately the same as an assembling pressure in assembling a fuel cell stack by stacking plural membrane electrode assemblies. In this feature, the membrane electrode assembly is pre-compressed with the assembling pressure in forming the seal before assembling the fuel cell stack, so that variation of the thickness thereof can be reduced, and the variation of the compressing thickness can be reduced. Therefore, the clamping thickness of the seal need not be excessive, and the size of the seal can be set in a suitable range.
As materials for the seal in the invention, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, may preferably be used. The seal can be adhered to the electrolytic membrane simultaneously forming the seal without adhering treatment such as primer treatment by selecting the same type material for the seat as the electrolytic membrane according to the kind of the material for the electrolytic membrane. When rigid adherence is required, adhering components can be added to the seal. For example, fluorine rubbers and elastomers of the fluorine type can be used for sealing materials when the electrolytic membrane is of the fluorine type.